Cathode structure



June 23, 1959 55355 2,892,120

CATHODE STRUCTURE Filed 001;. 22. 1954 James E. 542325,

- by 4. m

[is Attorney.

CATHODE STRUCTURE James E. Beggs, Schenectady, N.Y., asssignor to General Electric Company, a corporation of New York Application October22, 1954, Serial No. 463,849 7 Claims. (Cl. 313-343) This invention relates to cathode structures and methods of making such structures. While this invention is subject to a number of variations and modifications, it is suited to the formation of directly heated filamentary cathode structures and will be particularly described in this connection.

Electric discharge devices for use at high frequencies must have relatively small operating surfaces and electrode clearances in order to avoid inherent limitations at high frequencies such as self-inductive and capacitive reactance. The resulting relatively small size of electric discharge devices designed for high frequency operation results in a decrease in the permissible tolerances in the electric discharge device assembly and among other things the necessity of an electron emissive surface which is capable of supplying a high current per unit area and therefore capable of operating at sustained high temperatures.

At high frequencies it is also particularly desirable that the high frequency currents supplied to such a directly heated electron emissive element have a low inductance path through the heater leads and further that the electron emissive surface be uniformly and rapidly heated Without changing position relative to the other active surfaces of an electric discharge device throughout the range of operating temperatures.

It is therefore an important object of this invention to provide an improved cathode structure.

It is also an object of this invention to provide'a quick heating low inductance cathode structure.

It is also an object of this invention to provide an efficient uniformly and quickly heated cathode structure, provided with a symmetrical heater connection, which remains in the desired orientation with respect to the other operating surfaces of an electric discharge device throughout the entire range of operating temperatures.

According to an embodiment of this invention. a cathode structure is provided having a plurality of small diameter conductive filaments secured to a frame member. A conductor is connected to each of the filaments at a region intermediate the ends of the filaments so that the frame and the conductor provide a substantially symmetrical heater connection to the filaments.

The other objects and important aspects of this invention will become apparent from the following specification and claims when taken in connection With the figures of the drawing in which Figure 1 illustrates a plan view of a filamentary cathode constructed in accordance with this invention; Figure 2 illustrates a view of the showing in Figure 1 taken along section 2-2 and Figure 3 illustrates a portion of a completed cathode structure constructed in accordance with this invention.

Figure 1 illustrates a frame member 10 which may, for example, consist of a fiat disk of tungsten through which an opening or hole 11 is punched. A plurality of small diameter filaments 12 of, for example, tungsten Wire, extend across opening 11 and are suitably bonded 2,892,120 Patented June 23, 1959 to the surface of frame 10. A conductive member 13 is electrically and mechanically connected to the midpoint of each of the filaments and is electrically and mechanically coupled to heater lead 14.

A filamentary cathode such as that illustrated in Figures 1 and 2 can conveniently be formed by placing two tungsten disks such as tungsten .disk 10 back to back. A tungsten filament and the exposed surfaces of the tungsten disks are then dipped in a copper sulfate solution to place a thin coating of copper on the surfaces prior to winding the filament about the disks. A desirable size of tungsten wire is in the order of 0.3 mil and this wire is wound under tension about the disks so that the tungsten filament extends a number of times across the opening 11 of each ofthe disk-s. The entire assembly is then heated .in a hydrogen atmosphere to a temperature in the order of the melting point of copper, i.e. approximately 1100 C}, to braze the tungsten filaments to the tungsten disk. The disks are then separated by severing the filaments extending between the disks s o as toprovide two disks having a plurality of tungsten filaments under tension extending across the .opening in the disk. Connector 13 and heater lead 14 are then bonded to the filaments, for example, by brazing, and the filaments processed, .forexample, with a triple carbonate coating so as to act as eifective electron emitters. In the practice of an alternative method connector 13 and heater lead 14 are brazed to the .filaments at the same time the filaments are brazed to the tungsten disks.

While this description has been limited .to the application of fine filaments to a tungsten disk, it is readily apparent that filaments of other materials such as molybdenum and any other materials or alloys suitable for forming the base .of an electron emissive cathode and having a relatively high tensile strength can be utilized and that the frame 10 can have otherconfigurations than the illustrated disk with the centrally oriented .rectangular opening therein. The frame member. may be formed .of any material having sufiicient conductivity which can withstand the high operating temperatures of miniature electric discharge dev'ices and to which conductive filaments can be bonded; however, it is generally desirable to .utilize a disk of thesame'material as that of the filaments, .or one having the same or similar expansion characteristics. a

It is apparent that the abovedescribed embodiment provides a plurality of taut filamentary members with a. substantially symmetrical heater connection. In the example herein described, it may be assumed that the heater current flows up through heater lead 14out through cathode filaments 12 to .the frame 10 and back to the heater power source. p

Figure 3 illustrates a low inductance-cathode structure wherein the cathode structure of Figures 1 and 2 are inserted in a portion of an electric discharge device such as, for example, those illustrated and disclosed in my Patent No. 2,680,824 and my copending applications, Serial Nos. 464,126 and 464,079, filed herewith, all of which are assigned to the same assignee as this application. The cathode frame member 10 is retained in metal cylinder 15 which surrounds and shields heater lead 14. The cylinder 15 provides a heater lead for carrying electrical energy to the frame 10, shields heater lead 14 and provides an inherently low inductance connection to the filaments.

The cylinder may, for example, be formed of molybdenum or titanium. It is apparent that a large variety of materials, such as those commonly used in electric discharge devices, may be utilized to support the filamentary portion of the cathode structure; however, a material such 3 as titanium or zirconium is particularly suitable since it serves the dual function of acting as a heater lead and shield as well as acting as an effective getter for gases in a completed electric discharge device.

The tightly stretched tungsten wires, particularly when supported by connector 13 and heater lead 14, remain properly orientated relative to the other active electric discharge device elements throughout the entire operating temperature range, which may vary from approximately room temperature to temperatures approaching red heat, i.e. in the order of 700 C. The filaments are inherently rapid heating and the heating is essentially uniform since the current flow is from the center of the filaments out to the frame member 10.

Cathodes constructed in accordance with this invention are capable of being operated throughout a wide range of heater voltages and of being heated to operating temperature in a very short period of time, in the order of one quarter of a second after heater power is applied.

A cathode structure such as herein described is particularly suited for utilization as a directly heated filamentary type cathode; however, it will be readily apparent that this structure can be utilized in connection with a separately heated cathode by placing the filaments in heat conducting relationship with an electron emissive surface.

While this invention has been described by way of example in connection with a specific embodiment, it is apparent that this invention is subject to a number of modifications and it is intended in the appended claims to cover all such modifications as come within the true spirit and scope of this invention.

What I intend to claim and protect by Letters Patent of the United States is:

1. A cathode structure comprising a conductive frame member having a substantially non-conductive interior portion, a plurality of electron emitting filaments tautly stretched and secured to said frame member to extend at least partially across said non-conductive portion, and a conductor electrically connected to the filaments at a region approximately centrally of said non-conductive portion.

2. A cathode structure comprising a conductive frame member, a plurality of small diameter conductive electron emitting filaments tautly stretched and. secured to said frame member to extend across at least one opening in said frame member, and a conductor electrically con nected to said filaments at a region near the mid-points of the portions of said filaments extending across said opening whereby said frame member and conductor provide heater connections to said filaments.

3. In an electric discharge device, a cathode structure comprising a conductive frame member, a plurality of small diameter conductive electron emitting filaments tautly stretched across at least one opening in said frame member and secured at their end portions to said frame member, and a conductor connected to said filaments at a region approximately at the center of said opening whereby said frame member and conductor provide substantially symmetrical heater connections and said filaments remain substantially fixedly oriented throughout the range of operating temperatures of the cathode structure.

4. In an electric discharge device a cathode structure comprising a conductive frame member having a central opening therein, a plurality of small diameter conductive electron emitting filaments tautly stretched across the opening in said frame member and secured at their ends to said frame member, and a conductor extending through the opening in said frame member and connected to the mid-points of each of said plurality of filaments whereby said frame member and said conductor provide symmetrical heater connections to said filaments which remain substantially fixedly oriented throughout the range of operating temperatures of the electric discharge device.

5. In an electric discharge device a cathode structure comprising a conductive frame member, a plurality of small diameter conductive electron emitting filaments tautly stretched and secured to said frame member and extending across at least one opening in said frame mem her, and a conductor extending through an opening in said frame member and connected to each of said filaments at a region intermediate the ends of said filaments at approximately the center of said opening, a conductive shield coupled to said frame member and surrounding said conductor to provide a low inductance cathode structure.

6. A low inductance directly heated cathode structure comprising a conductive planar frame member having an aperture therein, a plurality of filamentary cathode wires tautly stretched across said aperture and attached to said frame member at the ends thereof, a conductor electrically connected to each of said filamentary cathodesat the center of said aperture and concentric conductors connected respectively with said last-mentioned conductor and said frame member and providing low inductance terminals for said cathode structure.

7. A lowinductance directly heated cathode structure comprising: a conductive frame member having an interior aperture therein, -a plurality of filamentary cathode wires tautly stretched across said aperture and attached to said frame member, and a conductor electrically connected to said filamentary cathode wires at a region over said aperture to provide a conductive path thereto.

- References Cited in the file of this patent UNITED STATES PATENTS 

